1. Field of the Invention
The present invention relates to a process for the production of metal nanoparticles. In particular, it relates to a process which affords better control of the size, size distribution and/or shape of the particles than the so-called polyol process.
2. Discussion of Background Information
The production of metal particles by the polyol process is known from, e.g., U.S. Pat. No. 4,539,041 to Figlarz et al., the entire disclosure whereof is expressly incorporated by reference herein. In the polyol process, a metal compound is reduced at an elevated temperature by a polyol to afford the corresponding metal in the form of particles (usually in the micron and nanometer size range). A number of metal compounds and in particular, a number of transition metal compounds can be converted to metal particles by this process. In a typical procedure, a solid metal compound is suspended in a polyol and the suspension is heated until the reduction of the metal compound is substantially complete. Thereafter, the formed particles are isolated by separating them from the liquid phase, e.g., by centrifugation.
A modification of this method is described in, e.g., P. -Y. Silvert et al., “Preparation of colloidal silver dispersions by the polyol process” Part 1—Synthesis and characterization, J. Mater. Chem., 1996, 6(4), 573-577; and Part 2—Mechanism of particle formation, J. Mater. Chem., 1997, 7(2), 293-299. According to the Silvert et al. articles, the entire disclosures whereof are expressly incorporated by reference herein, the polyol process is carried out in the presence of a polymer, i.e., polyvinylpyrrolidone (PVP). In particular, the PVP is dissolved in the polyol and helps to control the size and the dispersity of the metal particles. In a typical experiment, about 10 g of PVP was dissolved at room temperature in 75 ml of ethylene glycol and 2.4 mmole (400 mg) of silver nitrate was added to this solution. The resultant suspension was stirred at room temperature until the silver nitrate had dissolved completely, whereafter the system was heated to 120° C. and the reaction was conducted at this temperature for several hours. After cooling and dilution with water, the reaction mixture afforded silver particles having a mean particle size of 21 nm with a standard deviation of 16%.
While the reported results are desirable, the present inventors have found that when the modified polyol process is scaled up and conducted with a significantly larger amount of metal compound such as, e.g., 0.1 mole of metal compound or more, in order to produce metal nanoparticles in commercially significant amounts, the size and shape of the particles becomes non-uniform and the formation of large chunks, needle-like particles and the like is observed in addition to the formation of sphere-like particles. Accordingly, it would be advantageous to have available a process of the type described by Silvert et al. which affords satisfactory results in terms of particle size, shape and/or size distribution even when it is conducted on a significantly larger scale than that reported by Silvert et al. Corresponding nanoparticles would be useful in variety of applications. For example, there exists a need for compositions for the fabrication of electrically conductive features for use in electronics, displays, and other applications. Accordingly, nanoparticles produced by a process that affords commercially significant amounts of substantially non-agglomerated, redispersible metal nanoparticles with a substantially uniform shape and size could, for example, be used for the manufacture of printing inks and in particular, into inks for the printing of electrically conductive features.